Abstract: AIM: To study the effects of dl-, l- and d-3-n-butylphthalide(NBP) on pial arteriole diameter(AD) and blood flow velocity(BFV) in focal ischemia rats. METHODS: Urethane-anesthetized rats were instrumented for monitoring pial AD and BFV in the cranial window preparation. The effects of dl-, l-, d-NBP on AD and BFV were investigated in these left middle cerebral artery occluded(L-MCAO) rats by using the method of acute cranial window technique under in vitro videomicroscope. dl-,d-,l-NBP(25 mg.kg^-1 ip) and nimodipine(0.3 mg.kg^-1) were administrated systemically 20 min after MCAO or 1 h before MCAO. RESULTS: In the vehicle group, MCAO induced a significant decrease in BFV and AD, the levels of BFV and AD were reduced to 18.3% and 52% compared with the preischemia baseline values. In the pretreatment groups, no change in pial AD was found after dl-, l-, d-NBP administration in normal animals, and a rapid and marked decrease in BFV and AD of the selected pail artery was observed within 5 minutes after MCAO. The decreased level of AD and BFV recovered quickly after MCAO in the dl-, l-NBP and nimodipine groups, while the dysfunction of microcirculation was exacerbated by d-NBP. In the post-treatment groups, dl-NBP(12.5, 25 mg.kg^-1 ip) induced dilation of the pial arterioles and the increase in BFV was in dose-dependent manner. The pial arteriolar response to MCAO was not affected by d-NBP and nimodipine. CONCLUSION: These data suggest that the improving effects of dl- and l-NBP on microcirculation dysfunction during ischemia may play an important role in their protective effects against focal cerebral ischemia injury. l- and d-NBP showed counteractive effects on pial AD and BFV in MCAO rats indicating that NBP has stereoselective character on its protective action against cerebral ischemia injury.